Modeling the Potential of Submarine Groundwater Discharge to Facilitate Growth of Vibrio cholerae Bacteria

Round  1

Reviewer 1 Report

This study is a theoretical evaluation of the habitat availability for the Vibrio cholerae bacteria generated by submarine groundwater discharge. Considering the world is currently considered in a cholera pandemic and that millions of people are affected yearly by this disease, this work is potentially very important.

However, there are three drawbacks to this work that in my opinion prevents its publication. 1) The links to the life cycle of the bacteria are very weak; 2) The effects of tides and waves on salinity at the sediment-water interface were not considered; 3) No field evaluation was performed and no evidence provided the bacteria can realistically use SGD zones as a natural reservoir.

Whilst not a microbiologist, I understand Vibrio cholerae's natural habitat to be estuaries, where they can either occur in the water column (often associated with copepods) or in a dormant state in sediments. However, it is not clear here if the authors are evaluating the potential occurrence of the bacteria outside of estuaries. Even if SGD does generate suitable salinity conditions at the groundwater - surface water interface, it does not mean a suitable habitat has been generated, in particular if the life cycle includes a zooplankton. The authors need to clearly explain how a potential natural reservoir for the bacteria could be generated via SGD as the evidence they presented was very thin.

2) The basis for the assessment is that SGD can generate brackish conditions in an otherwise saline environment, where Vibrio is thought less likely to thrive. Various hydrogeological factors were considered to evaluate the potential to create the right salinity at the sediment surface via SGD. These scenarios were evaluated using a numerical hydrogeological model. This is hardly new because this topic has been covered at length in the literature. However, most work in this area currently considers the effect of tides on the generation of a tidal recirculation cell at the shoreline (as shown in many of the references listed by the authors). This would in general considerably reduce the area with a suitable salinity at the sediment surface for Vibrio (by contracting the area of freshwater discharge near low tide). Waves further 'salinise' freshwater SGD by inducing mixing with the subsurface - a process somewhat similar to hyporheic exchange in rivers. Thus, the conclusion reached by the authors that dispersion had little impact on the salinity distribution is almost certainly incorrect. One would anticipate that near the sediment-water interface dispersion is much greater due to waves and would in most cases 'salinise' the discharging groundwater.

You could argue the approach taken by the authors is conservative. However, it also brings the risk to considerably overestimate the potential habitat generated by SGD for V. cholerae, which is not ideal if the aim of the work is to help develop practical policies to control cholera outbreaks.

3) Whilst it is known V. cholerae is found naturally in estuaries, it seems less clear what other natural habitats it may inhabit. It would have been useful for the authors to demonstrate that V. cholerae have been found in SGD outside of estuarine environments or at least show some evidence why we should anticipate so. As discussed above and also acknowledged by the authors, salinity is only one of many factors controlling the distribution of this organism.

Author Response

Please see our response to the Reviewer 1 in the uploaded word file.

Author Response File: Author Response.pdf

Reviewer 2 Report

This study reports use of a model to simulate the impact of submarine groundwater discharge (SGD) on habitat for cholera bacteria in the marine environment.  The subject is novel (as far as this reviewer knows), and could be of interest in epidemiology, though it is entirely theoretical and not compared with field measurements of either SGD, habitat, or cholera presence.  A groundwater model is used to quantify the cross-sectional length of a theoretical sub-marine coast that is within the salinity tolerance limits of cholera.  Various values of coastal slope and saturated hydraulic conductivities (K) were used.  The model is not validated or compared against field measurements; this is rather a sensitivity analysis to identify major controls on habitat.  Overall, the study appears to be competently done, and provides an interesting perspective on the controls of cholera habitat.  It could be published with moderate revisions.

I think the motivation could be stronger.  Why do we need to know about the spatial distribution of cholera in the marine environment?  Is marine exposure a common way that outbreaks start?  Have others simulated cholera distribution in the marine environment? 

I found parts of the results section difficult to understand (see comments in the pdf).  The results section could use rewriting, especially lines 188-201.

One interpretation I found puzzling was (L 254) “The model shows that there is no clear coherence between V. cholerae habitat zones and volumes of SGD”.  But Figure 3 shows clear increases in habitat zone length and inflow volume.  Clarify.

See the annotated pdf with numerous additional comments, including several places that need re-writing for clarification.

Comments for author File: Comments.pdf

Author Response

Please see our response to the Reviewer 2 in the uploaded word file.

Author Response File: Author Response.pdf

Round  2

Reviewer 1 Report

I thank the authors for addressing some of the issues raised in my first review. I have given much thoughts about the validity of this study since the original review. The topic is interesting and important, that is fine. However, I am still concerned about the approach used to characterise the risk for SGD to generate Vibrio habitat along coastlines (via a suitable salinity regime). The modelling approach used is not new and there are many similar studies in the literature already. The authors have not shown that what they have found is different than similar previous modelling exercises. In addition, coastal environments are complex and estimating salinity at the groundwater - surface water interface especially difficult. Thus, it is not clear whether any of these modelling results are realistic and thus useful.

Because it is early on for the evaluation of this problem, a more empirical approach is required. Firstly, the modelling literature investigating salinity at the gw-sw interface should have been reviewed and these results interpreted in the context of Vibrio habitat. Secondly, the literature also includes numerous coastal studies measuring salinity at the gw-sw interface - how often do these reveal salinity conditions suitable for Vibrio? Under what conditions? This in my opinion would have been far more useful than (another) modelling study on salinity in this environment.

I encourage the authors to revisit the problem with an empirical rather than a modelling approach.    

Author Response

We thank the reviewer for the constructive and thoughtful comments. Indeed we agree that an empirical study of this topic would be of high value, yet the published literature that can be used to address this topic is less dense than can be expected just counting publications on the topic. The reported pore water salinity data are often difficult to interpret. Yet we surely will take on the challenge to revisit the problem with an empirical approach in the future.
For the current study we reviewed the SGD literature based on the literature corpus of existing reviews (e.g. Moosdorf et al., 2015; Cho et al., 2018) and refer to locations where based on the published data
the V. cholerae habitat conditions are reached at porous SGD sites. We included two studies to underscore that such conditions are found nature.
References cited here:

Cho, H.-M., Kim, G., Kwon, E.Y., Moosdorf, N., Garcia-Orellana, J.,
Santos, I.R., 2018. Radium tracing nutrient inputs through submarine
groundwater discharge in the global ocean. Scientific Reports, 8(1):
2439-2446.
Moosdorf, N., Stieglitz, T., Waska, H., Durr, H.H., Hartmann, J., 2015.
Submarine groundwater discharge from tropical islands: a review.
Grundwasser, 20(1): 53-67.